51st Lunar and Planetary Science Conference (2020) 2625.pdf

THALASSA: A MISSION TO FOLLOW THE WATER ON VENUS. Paul K. Byrne1, Elizabeth A. Frank2, M. Darby Dyar3, Jörn Helbert4, Peter Illsley2, Attila Komjathy5, Siddharth Krishnamoorthy5, Robert J. Lillis6, Joseph G. O’Rourke7, Emilie M. Royer8, Sean C. Solomon9, Constantine Tsang10, Christopher Voorhees2, and Colin F. Wilson11, 1North Carolina State University, Raleigh, NC 27695, USA ([email protected]); 2First Mode, Seattle, WA 98121, USA; 3Mount Holyoke College, South Hadley, MA 01075, USA; 4German Aerospace Center (DLR), Berlin, Germany; 5Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91125, USA; 6UC Berkeley Space Sciences Laboratory, Berkeley, CA 94720, USA; 7Arizona State University, Tempe, AZ 85281, USA; 8Planetary Science Institute, Tucson, AZ 85719, USA; 9Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA; 10Southwest Research Institute, Boulder, CO 80302, USA; 11Oxford University, Oxford OX1 3PU, UK.

Introduction: There are tantalizing indications that ratio and noble gas concentrations, to gain new insight Venus might once have possessed much more water into the extent of outgassing of the planet’s interior; than it does today [1–3], raising the prospect that the 3) Search for current evidence of volcanically second planet may have long been Earth-like and thus emitted water on Venus by mapping the abundance of even habitable. But when and how this water was lost, water in the lower atmosphere to ascertain whether whether it ever existed in liquid form on the surface, outgassing is occurring today; and whether geological or geochemical evidence of this 4) Document the role of water in the formation of water remains preserved on the surface, and even tesserae, to test whether this distinctive terrain type whether Venus and Earth really did start off with similar formed in the presence of abundant quantities of water water inventories remain unanswered questions [4]. and is thus analogous to Earth’s continents. Understanding the extent to which Venus was once Investigation 1—Water Loss: The D/H ratio in the a water world will substantially improve our atmosphere of Venus is about 100 times that of Earth understanding of this enigmatic planet, of the geological [e.g., 7]. This enrichment in deuterium can be evolution of our own world and the inner Solar System interpreted as either the signature of a lost primordial more broadly, and of the rules that govern Earth-size ocean of at least 0.3% the volume of an Earth ocean, or worlds in this and other planetary systems. Our mission of a steady state in which water is continuously supplied concept is named for the primordial spirit of the sea in to the surface of Venus by comets or volcanic Greek mythology and traces back to our singular outgassing, balancing loss through hydrogen escape [8]. scientific goal: follow the water on Venus. Nevertheless, the extreme aridity of Venus’ current Scientific Goal: The Thalassa scientific goal is atmosphere (with a water vapor content of 0.003%) arguably the highest-priority goal for Venus exploration compared with Earth led numerous workers to conclude and encompasses climate, geology, and geophysics. that large amounts of water have been lost from Venus Water is the key to Earth’s habitability [e.g., 5] and since its formation [e.g., 7]. likely plays a major role in plate tectonics [4]. But major uncertainties remain about atmospheric Therefore, water is a vital clue to the mystery of whether loss mechanisms at Venus. Acquiring well designed and early Venus and early Earth were similar, and, if so, continuous observations of the Venus upper atmosphere when and why the geological evolution of Venus and will provide vital information on the rates at which H Earth diverged. Thalassa’s scientific goal is directly and O, and thus water, are lost to space at present traceable to NASA’s strategic objectives for planetary (Figure 1a). Such observations will contribute to a science and addresses high-priority questions from all comprehensive understanding of how hydrogen and three broad, crosscutting themes in the 2011 “Vision oxygen atoms are transported through the bulk of the and Voyages for Planetary Science in the Decade 2013– atmosphere before escaping from the upper atmosphere, 2022” Decadal Survey. The Thalassa scientific goal also and will act as invaluable constraints for global directly addresses many high-priority investigations circulation models. from the 2019 Venus Exploration Analysis Group Investigation 2—Outgassing: The middle and (VEXAG) report, “Scientific Goals, Objectives, and lower atmosphere of Venus contain critical information Investigations (GOI) for Venus Exploration” [6]. on the past and current outgassing history of the planet. Scientific Investigations: Stemming from our Determining whether liquid water was ever present can “follow the water” goal, the Thalassa mission concept be in part answered by characterizing atmospheric features four scientific investigations that explore the species in the modern bulk atmosphere. By doing so, we role of water from the upper atmosphere to the surface. can address such outstanding questions as how much Ordered here from greatest to lowest altitude at Venus, water was in the interior of Venus in the past, when those objectives are: these outgassing events occurred, and if and when 1) Characterize atmospheric water loss at Venus, outgassing and water loss ended. including escape rates of H and O and atmosphere–solar Measuring isotopic ratios of atmospheric noble wind/plasma environment interactions; gases such as He, Ar, Kr, and Xe will allow us to 2) Determine the history of Venus’ outgassing and constrain the sources and sinks of the past atmosphere water loss, including its deuterium–hydrogen (D/H) (Figure 1b). Because these noble gases would have 51st Lunar and Planetary Science Conference (2020) 2625.pdf

outgassed together with large reservoirs of water vapor, measuring the abundances of these gases will help us understand when and how much water was injected from the interior to the surface and atmosphere, offering a critical insight into Venus’ primordial water content. Investigation 3—Volcanic Activity: The Venus deep troposphere (from ~15 km to the surface) contains a minute abundance of water vapor; as yet, no spatial or temporal variability has been detected [9]. On Earth, volcanic eruptions are a leading mechanism by which water vapor is injected into the atmosphere, with modern volcanic plumes containing large amounts of H2O [e.g., 10]. Water vapor plumes should be easier to detect on Venus than on Earth because of the low background variability of tropospheric water vapor abundances. By studying near-surface atmospheric water vapor concentrations we will search for evidence of recent or ongoing volcanism, with the main objective of determining whether Venus’s interior contains amounts of water comparable with Earth (Figure 1c). Investigation 4—Tessera Formation: Tessera Figure 1: (a) Thalassa will characterize atmospheric loss terrain occupies ~8% of the planet’s surface and is mechanisms at Venus to an unprecedented precision. (b) By interpreted as the locally oldest material on Venus [11] accurately measuring middle-atmosphere composition, (Figure 1d). Tesserae are tectonically complex Thalassa will establish Venus’ degassing history. (c) Thalassa will search for evidence of injections of water into the geological units with styles of deformation not observed atmosphere, helping to answer whether Venus is volcanically on any other planetary body (other than, perhaps, the active. (d) Are the tesserae akin to Earth’s continental continents on Earth). These units occur as both large materials? Thalassa will tackle this question head on. plateaus thousands of kilometers across and as smaller inliers (isolated, high-standing regions hundreds of characterize the bulk composition of the middle Venus kilometers in breadth that are embayed by later younger atmosphere for Investigation 2. And a set of volcanic deposits) [11]. miniaturized, NIR-instrumented dropsondes will deploy On the basis of morphology, gravity signature, and from the aerial platform to target a subset of the tesserae inferred composition, tesserae have been proposed to be resolved by the orbiter for Investigation 4. the Venus counterparts of continents on Earth [12–14]. Our proposed combination of orbiter, aerial Although fractional crystallization can yield felsic rocks platform, and dropsondes represents a novel approach from mafic (or ultramafic) source rock, this process is to Venus exploration on the scale of a New Frontiers highly unlikely to account for the estimated volume of mission. Venus offers us a natural laboratory in which tesserae on Venus [e.g., 15]. This conclusion bolsters to understand how large rocky planets form and the hypothesis that the tesserae are analogous to granitic evolve—so isn’t it time we went back? continental materials on Earth, which form via crustal References: [1] Treiman A. H. (2007) in Exploring Venus as recycling in the presence of substantial volumes of a Terrestrial Planet, Esposito L. W. et al. (eds.) AGU Geophys. subsurface water [16]. Because felsic rocks have Monograph Ser., 176, 7–22. [2] Elkins‐Tanton L. T. et al. (2007) proportionately more silicon and less iron than silica- JGR, 112, E04S06. [3] Way M. J. et al. (2016) GRL, 43, 8376– 8383. [4] Smrekar S. E. et al. (2007) in Exploring Venus as a poor mafic rocks, characterizing the total iron Terrestrial Planet, Esposito L. W. et al. (eds.) AGU Geophys. abundances of Venus’ tessera terrain offers a direct Monograph Ser., 176, 45–71. [5] Tosi F. et al. (2017) EGU measure of whether they are compositionally similar to General Assembly, 19, abstract 7375. [6] VEXAG Goals, continental materials on Earth [17]. Objectives, and Investigations for Venus Exploration (2019). [7] Notional Implementation: An instrument suite Donahue T. M. et al. (1982) Science, 216, 630–633. [8] based on MAVEN’s heritage [e.g., 18] and carried on Grinspoon D. H. et al. (1993) PSS, 41, 515–542. [9] Haus R. et al. an orbiter element will address Investigation 1. The (2015) PSS, 105, 159–174. [10] Burton M. E. at al. (2001) JGR, orbiter will also search with a near-infrared (NIR) 106, 30,099–30,107. [11] Ivanov M. A. & Head J. W. (2011) PSS, spectrometer [cf. 17] for evidence of recent water 59, 1559–1600. [12] Hashimoto G. L. & Sugita S. (2003) JGR, 108, E002082. [13] Hashimoto G. L. et al. (2008) JGR, 113, injection into the lower Venus atmosphere for E003134. [14] Romeo I. & Capote R. (2011) PSS, 59, 1428–1445. Investigation 3, and will characterize the iron content of [15] Gilmore M. S. at al. (2017) SSR, 212, 1511–1540. [16] major tessera units across the surface for Investigation Campbell I. H. & Taylor S. R. (1983) GRL, 10, 1061–1064. [17] 4. An aerial platform, taking inspiration from the Soviet Dyar M. D. et al. (2018) VEXAG Meeting, 16, abstract 8015. [18] VeGa balloons (which explored Venus’ clouds in 1985) Jakosky B. M. (2018) Icarus, 315, 146–157. [19] Cutts J. A. et al. but incorporating contemporary capabilities [19] will (2018) Venus Aerial Platform Study, JPL D-102569.